1. See, for example, N. Isgur and G. Karl, Phys. Rev. D18 4187 (1978).
2. H. Georgi, Weak Interactions and Modern Particle Theory (Benjamin/Cummings, Menlo Park, CA, 1984).
3. A. De Rujula, H. Georgi and S. Glashow, Phys. Rev. D12 147 (1975).
4. G. ’t Hooft, Nucl. Phys. B72 461 (1974).
5. E. Witten, Nucl. Phys. B160 57 (1979)